Oscillators are widely utilized which use quartz crystal units to generate reference frequencies of electronic devices. When cut at a specific angle from crystalline quartz and resonated in a specific mode, the quartz crystal unit can significantly reduce temperature dependency of a resonance frequency. Typical examples such as AT cut TS (thickness-shear) mode resonators and X cut two-legged tuning fork vibrators are widely utilized.
While the quartz crystal unit has good temperature characteristics, growth of crystalline quartz involves many drawbacks of requiring several months in a high temperature and pressure container, which is called an autoclave, and increasing cost, requiring highly precise processing, requiring mechanical mounting process in an air-tight package to form a resonator and also having a limit in miniaturization of the dimension due to mechanical processing.
Hence, a Si vibrator created by the MEMS technology is recently gaining significant attention. A Si resonator is made on a Si substrate in semiconductor process, so that it is possible to ultra-miniaturize the Si resonator or reduce cost of the Si resonator. Further, by using single crystal Si or sufficiently annealed poly crystal Si as a resonator, resonance can be carried out very stably and a Q value of resonance is also high.
However, unlike a quartz crystal unit, the Si vibrator has temperature dependency of the resonance frequency of about −30 ppm/° C. The temperature dependency mainly derives from temperature dependency of an elastic modulus of Si. An error is about 0.3% at −30° C. to 85° C. which is an operating temperature limit of consumer equipment. Hence, reference frequency oscillators cannot be used without performing some temperature compensation in almost all applied fields.
For this problem, a method of compensating for the temperature according to digital processing at a later stage of a Si oscillation circuit is proposed. That is, apart from a first oscillation circuit having a Si vibrator, a second resonance circuit formed with an inductor and a variable capacitor, a temperature sensor and a PLL circuit are provided. The temperature is compensated for by controlling the frequency of the second resonance circuit at a frequency division ratio set in advance according to the temperature by means of the PLL, based on the resonance frequency of the Si vibrator.
Although this method can provide comparatively high temperature compensation precision, an output is obtained from the second resonance circuit and the second resonance circuit is formed with a LC tank circuit formed with an inductor and a variable capacitance capacitor, and therefore this method involves drawbacks of a small Q value and significant phase noise. Hence, this method cannot be actually applied to a field such as a field of mobile telephones which particularly require stability of oscillation.